Cast high explosive primer

ABSTRACT

A LENGTH OF DETONATING FUSE IS CUT, DOUBLED AND THE TWO ENDS PASSED THROUGH A SHORT LENGTH OF FLEXIBLE TUBULAR SLEEVE. THE CUT ENDS OF THE RESULTING LOOP OF DETONATING FUSE ARE THEN IMBEDDED IN A CUP OF MOLTEN HIGH EXPLOSIVE SO THAT, UPON COOLING, THE LOOP WITH SURROUNDING SLEEVE PROTRUDES FROM THE SURFACE OF THE CAST EXPLOSIVE, FORMING AN ASSEMBLY WHICH IS USEFUL IN ATTACHING A CONNECTING PIECE OF DETONATING FUSE OR IN POSITIONING A BLASTING CAP OR ELECTRIC DETONATOR SO THAT THE END CONTAINING THE BASE CHARGE IS MAINTAINED IN FIRM PHYSICAL CONTACT WITH BOTH THE DETONATING FUSE AND THE SURFACE OF THE CAST HIGH EXPLOSIVE.

Oct. 26, 197] PARTRIDGE ETAL 3,614,928

CAST HIGH EXPLOSIVE PRIMER Filed March 12, 1969 myENToRs 5. Pa /77v BY M1142.

ATTORNEY United States Patent O1 flee 3,614,928 Patented Oct. 26, 1971 3,614,928 CAST HIGH EXPLOSIVE PRIMER Dale S. Partridge, Shawnee Mission, Kaus., Allen Lemoyne Caldwell, Snowflake, Ariz., and Charles E. Tuttle, Jr., Mission, Kans., assignors to Gulf Oil Corporation, Pittsburgh, Pa.

Filed Mar. 12, 1969, Ser. No. 806,406 Int. Cl. F42b 3/10 US. Cl. 102-24 1 Claim ABSTRACT OF THE DISCLOSURE A length of detonating fuse is cut, doubled and the two ends passed through a short length of flexible tubular sleeve. The cut ends of the resulting loop of detonating fuse are then imbedded in a cup of molten high explosive so that, upon cooling, the loop with surrounding sleeve protrudes from the surface of the cast explosive, forming an assembly which is useful in attaching a connecting piece of detonating fuse or in positioning a blasting cap or electric detonator so that the end containing the base charge is maintained in firm physical contact with both the detonating fuse and the surface of the cast high explosive.

DESCRIPTION OF THE INVENTION Blasting explosives are ordinarily manufactured and packaged in a form in which they are not sensitive to the impacts and friction which can occur in the normal course of transportation and placement of charges. The cheapest and probably most commonly used blasting explosives at the present time are based on ammonium nitrate and a small percentage of a suitable fuel, such as a high-boiling petroleum distillate or a finey divided carbonaceous substance or oxidizable metal. The maximum detonation velocity of the common ammonium nitrate explosive is not very high, somewhere between 3,000 and 4,000 meters per second, whereas the common high explosive high velocities within the range of about 6,000 to 8,000 meters per second. Most of the common blasting explosives which are sufiiciently insensitive so they can be safely dropped or rammed into position have detonation velocities between 4,000 and 6,000 meters per second and explosive energies of about 500 to 1,000 calories per gram. This class of explosives was originally developed by Alfred Nobel, who also developed the technique of detonation of less sensitive explosives with small charges of explosives with higher detonation velocities and higher explosive energies. This has been accepted practice in the use of industrial explosives since about 1875.

Because of the very low detonation velocity and sensitivity of ammonium nitrate, there is a tendency for the compression wave of an initial detonation to become sufiiciently attenuated before it has traveled the full length of a charge, so that initiation is ineflicient. It is not uncommon for undecomposed ammonium nitrate to be blown out of a blast hole and likewise for the gases from the explosion to have a brownish color, indicating incomplete reaction. To overcome the low inherent detonation velocity and energy of ammonium nitrate it has become general practice to sensitize the ammonium nitrate with intimately mixed fuels or small percentages of other explosives. Other conventional techniques for aiding initiation, such as introduction of centers of heterogeneity (gas bubbles, barium sulfate, refractory grit, coarse granular metals, etc.) are also commonly employed. However, even with the use of these conventional techniques, it is advisable to employ a substantial quantity of a primer charge of high explosive so as to efficiently detonate the entire charge of ammonium nitrate explosive.

Primers, or boosters, as they are sometimes called, are customarily made of a major quantity of a high explosive and an initiating quantity of detonating fuse or PETN and with some provision for attaching thereto a detonating fuse, electric detonator or blasting cap. The combination of detonator and primer is designed to produce a compression wave of sufficient amplitude and total energy that it will travel through and initiate the entire charge of ammonium nitrate explosive. Since the primer is composed of expensive ingredients, it is necessary that this device be designed to operate efiiciently, with the minimum quantity of high explosive. In simplest terms, high efiiciency in a primer is obtained by producing a substantially simultaneous detonation of all of the explosive ingredients of both detonators and primer, so that the pressure peaks add together to increase the amplitude of the compression wave. This objective is not readily accomplished and for this reason improvement of the design of primers is continuously sought by the manufacturers of these devices. Some of the most eflicient primers in use at the present time either require tedious hand labor in assembly, or are excessively bulky, having been cheapened by extending the high explosive with grains of blasting powder or other lower cost ingredients.

We have invented a primer which requires a minimum of hand labor in assembly as well as a minimum quantity of expensive high explosives and because of certain mechanical features, can be exploded with very high efficiency. FIG. 1 is a longitudinal cross-sectional 'view through the center of the body of cast explosive in the primer, showing the location of the ends of a length of detonating fuse imbedded in the cast solid explosive. FIG. 2 is an overall view of the primer with an electric detonator positioned for use.

In manufacturing the primer, a length of detonating fuse such as primacord or cordtex is cut and doubled to form a loop 1. Over the cut ends there is then passed a flexible sleeve 2, which may be obtained by cutting oif a short length of flexible plastic tubing. The flexible sleeve is then placed in a clamp which holds the fuse and sleeve assembly in a stationary position after the cut ends of the fuse are immersed in molten high explosive contained in a cup-like container 3. Upon cooling, a solid mass of cast high explosive 4, is obtained with the ends of the detonating fuse firmly imbedded therein and having an exposed upper surface 5. The exposed upper surface of cast explosive is important so that when a detonator 6 is placed in position, as shown in FIG. 2, good contact is obtained between the cast explosive and the end of the detonator containing the base charge. The flexible sleeve is also essential to efficient functioning of the primer, as it provides a means to hold a detonator in close physical contact with both the detonating fuse and the cast explosive. The flexible sleeve also provides a means for quickly forming a tight knot between the end of a piece of detonating fuse and the fuse loop on the primer, if this is the chosen means of detonation.

EXAMPLE I In a specific embodiment of this invention a primer consists of a cylindrical cup approximately 50 mm. high and 50 mm. in diameter filled with cast pentolite (50 percent PETN, 50 percent TNT) in which is imbedded both ends of a piece of SO-grain primacord about 205 mm. in length, doubled into a loop and passed through a sleeve consisting of a 25 mm. length of polyethylene tubing having an inside diameter of about 13 mm. When employed in small diameter blast holes to detonate ammonia nitrate-fuel oil explosives, with electric detonator attached as shown in FIG. 2 the performance of this primer compared favorably with commercial primers containing more than twice as much high explosive.

3 EXAMPLE 2 In another embodiment the two ends of the primacord loop were tied together in an overhand =knot before insertion in the molten high explosive. This embodiment required the use of a longer piece of primacord. In addition to good detonation performance, this primer was judged to have good physical strength and improved resistance to damage from rough handling, the primacord being well anchored in the body of cast pentolite.

EXAMPLE 3 A primer containing one-fourth pound of cast pentolite was constructed as shown in FIG. 1 and to it was attached an electric detonator as shown in FIG. 2. The primer was placed on top of a mild steel plate of one-half inch thickness and was detonated, resulting in perforation of the plate, the hole being round and approximately the same size as the primer. A conventional one-third pound pentolite primer was equipped with an electric detonator, placed on the plate and fired, producing only a dent in the plate. It was necessary to use a three-fourth pound conventional type primer in order to perforate the plate.

EXAMPLE 4 A series of cast primers was prepared as shown in FIG. 1, with decreasing amounts of cast pentolite in the same size cup. The smallest quantity used filled the cup to a depth of only one-fourth inch. The ends of the detonating fuse were lowered to the bottom of the cup so as to obtain secure irnbedding in the molten explosive. All of the primers in the series detonated successfully. Upon comparison with commercial cast pentolite primers with a hole through the center for a loosely threaded piece of detonating fuse, it was found that when the primer was shorter than three inches, detonation with the same size 4 fuse was not reliable. This was attributed to poor physical contact between the two explosives.

The preferred shape of the body of cast high explosive in the primers of this invention is a cylinder in which the height is approximately equal to diameter. The flexible sleeve, which is essential for obtaining good contact between detonator, cast explosive and detonating fuse, may be made of a variety of materials. Although flexible plastic tubing is preferred, a short length of coiled wire or thin metal tubing may be used. If metal tubingis employed for this purpose, it is necessary to round off the edges of the metal at the ends of the sleeve so they will not chafe or cut the loop of detonating fuse.

It is recommended that the length of the loop of detonating fuse be shorter than the length of the electric detonator which will be used. With a shorter loop, the Wire leads can be passed down under the loop so that when placed under tension the detonator will be forced down against the exposed surface of the cast explosive, rather than pulled away from it.

I claim:

1. A primer for detonating ammonium nitrate explosive charges comprising a body of solid cast high explosive having imbedded therein both ends of a length of detonating fuse, said length of fuse being formed into a loop outside the body of cast high explosive and encircled by a movable flexible tubular sleeve.

References Cited UNITED STATES PATENTS VERLIN R. PENDEGRASS, Primary Examiner US. Cl. X.R. 10227 

